As electronic parts, electronic devices and the like trend toward higher densities, electroless gold plating is coming into broader use, particularly since it is applicable to fine wiring. Reasons for this include the fact that circuits required for electrolytic plating around the object to be plated aren't needed because it is electroless, and the fact that it contributes to low-cost, simplified processes.
However, cyanide baths have been used conventionally for electroless gold plating liquids because of their stability, raising concerns about damages to the resist. Moreover, the high toxicity of cyanide electroless gold plating liquids has led to demands for non-cyanide processes, but baths such as sulfite baths that do not contain cyanide, for example, tend to decompose easily, and improvements are needed. Under these circumstances, non-cyanide electroless gold plating liquids have already been proposed.
Japanese Patent Documents 1 to 3 disclosed that an effective way for both bath stability and gold deposition speed of a non-cyanide electroless gold plating liquid is to use a mixed sulfite-thiosulfate complex system as a gold complexing agent and to use an urea compound such as thiourea as a reducing agent. However, when thiourea is used as a reducing agent, it is oxidized and a non-water-soluble cyanamide polymer is produced, causing problems of plate deposition failure. Therefore, as effective countermeasures, the addition of amine and carboxylic acid components for the aim of watersolubilizing the oxidation intermediate and addition of aliphatic saturated alcohol to suppress the autolysis of the thiourea reducing agent are proposed.
Moreover, Patent Document 4 states that the addition of a mercaptobenzothiazole compound with the aim of suppressing contamination by metal impurities and improving bath stability is useful when using the aforementioned mixed complexing agent together with ascorbic acid as the reducing agent, however, the reduction efficiency by ascorbic acid is poor, which needs to be added in excess to ensure a practical deposition speed (Patent Document 5). Patent Document 5 suggests the complexing agent system as described above, an aromatic compound typified by hydroquinone as a reducing agent system, and a heavy metal compound such as thallium nitrate in order to improve the deposition speed, but, the use of heavy metals, such as thallium, is desired to be substituted with other safer substances from the view point of toxicity, like cyanide.    Patent Document 1: Japanese Patent Publication No. H9-287077    Patent Document 2: Japanese Patent Publication No. H5-78854    Patent Document 3: Japanese Patent Publication No. H11-12753    Patent Document 4: Japanese Patent No. 3148428    Patent Document 5: Japanese Patent Publication No. 2003-268559